Desert Holes

THE HOLE TRUTH Desert Burrows - and What Lives in Them Text by Pinau Merlin

Squatting beside a saguaro on a rocky hillside, we peered into a dark half-moon-shaped burrow under a rock ledge. “Here, use this mirror,” I offered. Len angled my small pocket mirror until it reflected the sun's rays into the burrow, which was about 7 inches across. He looked at me, astonished. “There's a tortoise in there!” “It's a desert tortoise burrow, so it's not too surprising to find a tortoise there,” I said, smiling. “The flat bottom and half-moon tortoise shape give it away every time.” I had recently finished writing A Field Guide to Desert Holes, and my friend Len wanted to try deciphering clues as to which critters had made all those desert holes and burrows. We'd started our quest early in the morning along a desert wash near Tucson, and now we were watching the tortoise, which showed no sign of moving. So we left the tortoise to its contemplations and picked our way down the hill, where we found a small hole under a cholla cactus. Since a hole's size gives a major clue as to which critter dug it, Len had brought a tape measure with him. On handsand knees, he measured the hole and checked the burrow's angle of inclination, another clue. This one, fairly shallow, sloped at less than a 45-degree angle.

"It's a little over 2 inches in diameter," he announced. "I'd guess it's too small for anything bigger than a rodent."

Since the desert abounds in rodents, we looked for other clues to narrow the choices. We spotted chewed-up bits of prickly pear cactus fruit on the ground near the hole and little divots dug in the soil nearby. The divots were only an inch or so deep, where the mystery critter had dug out buried seeds to eat. Len felt the cactus fruit pieces.

"Still very moist," he reported. "This is fresh, so it must be an animal that's active during the day." I nodded. Len was momentarily stumped.

"Let's sit on this rock a minute and see what might happen," I suggested.

Just about five minutes later, the answer appeared-a Harris' antelope squirrel with red stains around its mouth from eating prickly pear fruit. It ran by us and plopped down in the shade of a paloverde tree.

After we meandered along a trail awhile, Len pointed out a quarter-size hole near a brittlebush. A tiny collar of twigs and debris bound together with fine silken threads encircled the hole, which angled nearly vertically into the ground. A little mound of PRECEDING PANEL, PAGES 14 AND 15] The desert floor's permanent residents often make their homes in many curious holes of various sizes, like the one framing this tarantula. The trick is figuring out who lives where. LYNDA RICHARDSON [OPPOSITE PAGE, BOTTOM] Unable to dig, diamondback rattlesnakes like to "borrow" holes already dug. LYNDA RICHARDSON [OPPOSITE PAGE, TOP LEFT] Desert tortoises also sometimes use existing shelters, but have no problem digging their own if necessary. MARTY CORDANO [OPPOSITE PAGE, TOP RIGHT] You can often see the hairylooking wolf spider when walking in the desert, but usually not this close. MARTY CORDANO [LEFT] Back safely in his den, the wolf spider intensifies his menacing glare. LYNDA RICHARDSON dirt stood a few inches away from the hole. "Must be an insect hole, though I guess a mouse could be that small," he mused. As we examined the hole with a hand lens, Len noticed the silk. "A spider!" he guessed. "Yes, a wolf spider," I agreed. "They always make that collar of twigs around the hole. This one has either just enlarged the hole or it's a new burrow, freshly excavated. See the little pile of dirt there where the spider has dumped the soil it pulled from the burrow? "Tarantulas make a hole like this too, but without the collar of twigs. Theirs often have a film of silk over the surface of the hole during the day." Thrilled, Len insisted we come back after dark to watch the spider hunt. We hadn't walked much farther when I grabbed Len and yanked him to a stop. He'd almost stepped on an oval depression in the dirt with a ridge around it-a rattlesnake sleeping form. When a snake coils up to rest, its body pushes little pebbles and twigs out of the depression and creates a ridge of dirt around the form, which is usually about 7 to 9 inches in diameter. Although no rattlesnake slept in the indentation at the moment, it appeared so fresh that the snake couldn't be too far away. "How do you know the form is fresh?" Len asked. I pointed out how sharp and crisp the dirt Ridge was and that there was no evidence of bird or rodent tracks crossing the form. Had it been there very long, birds like quail or thrashers already would have come to use such a nicely smoothed-out, soft spot in the dirt for their dust baths. We were trying to pick up the snake's track when we heard a raucous gang of cactus wrens calling from a catclaw acacia tree just off the trail. "There's our snake," I said. "Let's go see." "What are you talking about?" Len challenged. "How do you know the snake is over there?" "Listen to all those cactus wrens. They don't like snakes and usually mob them when they find them, and that's what they're doing right now." The medium-size diamondback rattler lay curled up in the mottled shade under the catclaw. The wrens, too fearful to approach the snake or attack it, hurled insults at it from the safety of the tree's thorny branches. Their alarm calls would alert other birds and animals to its presence, probably reducing the snake's chances of getting a meal. The wrens flew off at our intrusion. "What will the snake do now that the animals know he's here?" Len asked. "As the day warms up and gets too hot for him, he'll go down into a pack rat house or other convenient burrow to sleep through the day and then come back out to hunt in the evening." "You mean they don't have their own burrows?" he asked. "I thought half the holes in the desert were snake holes." "Lots of people think that," I agreed, "but most snakes have nothing to dig with to make their own burrow. They're borrowers, using any handy burrow they find as they move about. They especially like pack rat houses because they're like bed and breakfasts-a tasty pack rat and a place to sleep. I'll bet there's a pack rat house around here somewhere." Len found a nearby clump of prickly pear cactus with a mound of sticks, cholla joints, mesquite beans and debris at its base. We could see three entrances right at the house and a back door hole about 8 feet away. "I guess if you live at the bottom of the food chain, you want a few back doors for quick escapes," Len decided. The day was heating up, so we headed back to the car. Len shook his head. "It's like reading a mystery novel with a horde of suspects. You have to know how to read all the clues to discover the 'hole' truth." AN Pinau Merlin is a naturalist, author of the books A Field Guide to Desert Holes and A Guide to Southern Arizona Bird Nests and Eggs and a contributor to A Natural History of the Sonoran Desert. She lives in Tucson.